Oil-flooded screw compressor with axial-thrust balancing device

ABSTRACT

In an arrangement on an oil-flooded screw compressor with two rotors, a male rotor having essentially convex lobe flanks and a female rotor having essentially concave flank portions, with the male rotor having a drive-shaft end and an oil-pressure loaded balancing piston supplied with oil via a pressure line, both rotors are enclosed in housing sections, with a regulating device arranged either on or in the compressor the inputs of which are at least connected to measuring means for the pressure on the suction side and measuring means for determination of the input power, wherein according to the invention means for changing the pressure in the pressure line to the balancing piston are arranged with said means having a control interconnection to the regulating device, with the latter having an algorithm between operating parameters representing input variables for the regulating device and the pressure in the pressure line to the balancing piston.

The invention relates to an arrangement on oil-flooded screw compressors with two rotors, a male rotor having essentially convex lobe flanks and a female rotor having essentially concave flank portions, with a rotating disk, the balancing piston, arranged on a shaft section of the male rotor for balancing the axial thrust and sealing contactlessly and being loaded by pressurized oil which is drained to areas of lower pressure after passing the sealing gap of the balancing piston and which counteracts the gas force on the male rotor in axial direction. Both rotors are enclosed in housing sections. The profile sections of the rotors have shaft shoulders enclosed in radial bearings with the axial forces being supported by axial bearings. The gas force acting in axial direction of the rotors is comparatively high at the male rotor compared to the female rotor. For this reason, the male rotor features a balancing piston. The force on the axial bearing of the male rotor is a resultant from the difference of the gas force acting on the rotor and from the unloading force acting on the balancing piston. The working space designated also as working chamber is formed by the interlobe spaces of both rotors, adjacent housing sections and other adjacent components such as a control slide. Depending on the position of the control slide circumferential wall portions of the cylindrical sections enclosing the rotors are opened. Working fluid already sucked in is shunted back via a channel system in the housing to the suction side through the opened circumferential wall portions out of the interlobe spaces decreasing due to rotation of the rotors. This has an influence on the compressor displacement and on the axial force on the bearings.

Depending on the suction pressure, discharge pressure, size of the discharge port and the position of the control slide, the resultant from the difference of the gas force acting on the rotor and from the unloading force acting on the balancing piston will change. The resultant will act in one or the other direction depending on the operational condition of the compressor.

As a result, the axial bearings will be loaded more or less in one or the other direction. In this case, in the area of the axial bearing gap and the elastic deformation of the bearing, there will be a change of the distance between the male rotor and the end faces of the housing walls on both the suction- and discharge sides of the rotor profile. During part-load operation, the unloading force can also considerably exceed the gas force on the male rotor. For this reason, the unloading action of the balancing piston during part-load operation of the compressor is cut off in a known technical solution.

A disadvantage is that in this case the entire gas force in a greater part-load region will act on the axial bearings, thus shortening impermissibly the service life of the axial bearings.

The object of the invention is to prevent the disadvantages mentioned and to create an arrangement on a screw compressor enabling the bearing loads to be held on a constant low level.

According to the object of the invention, the oil pressure on the balancing piston, and hence the unloading force of the balancing piston, is controlled depending on operating parameters. The output signal of a regulating device preferably controls the pressure on the balancing piston depending on the driving torque at the compressor drive shaft and depending on the pressure on the compressor suction side. For this purpose, the output signal of the regulating device in a first arrangement according to the invention acts on a pressure-regulating valve located in an oil-supply line from a pressure source to the balancing piston. The control algorithm is designed so that the pressure on the balancing piston will increase when the driving torque rises or when the pressure on the suction side rises or when in combination of rising driving torque and rising pressure on the suction side there will occur changes. Preferably, there exists a control algorithm considering these parameters and calculating the required pressure on the balancing piston.

In a technically preferable embodiment according to the invention the means for realization of this control task represent a regulating device into which the speed of rotation to be expected, e.g. 49 1/s, the electric voltage of a three-phase asynchronous motor, e.g. 380 V, the cos φ, e.g. 0,93, and the electrical efficiency of the electric motor, e.g. 0,95, have been entered manually, or are metrologically registered during operation, analogue inputs for registration of the motor current, for registration of the pressure on the compressor suction side, as well as an output with an analogue signal representing the desired pressure on the balancing piston. Voltage, cos φ, efficiency and motor current serve in the algorithm of the regulating device first to determine the power at the compressor drive shaft, and in connection with the speed of rotation the torque will be calculated. In addition, the entire control algorithm considers the pressure on the compressor suction side during calculation of the output signal.

The output in a preferable arrangement according to the invention has a control interconnection to a pressure-regulating valve, e.g. a proportional pressure-regulating valve, with the flow rate being modulated by changing the output signal of the regulating device. Depending on the flow rate, the pressure on the balancing piston will change. The regulating device incorporates the control algorithm for calculation of the required pressure on the balancing piston and the presentation of this value in the range of a standard signal, e.g. from 4 to 20 mA, or directly as control current for a proportional pressure-regulating valve.

In another embodiment according to the invention, a pressure-measuring point is located in the supply line between the pressure-regulating valve and the connection for admission of pressure to the balancing piston. The output signal of the regulating device and the output signal of the pressure-measuring point are led to a three-position controller and compared. Depending on the result of the comparison, the proportional pressure-regulating valve will be opened more, when the pressure at the pressure-measuring point lies below the calculated value of the regulating device, or will be closed more, when the pressure at the pressure-measuring point exceeds the calculated value of the regulating device.

In another embodiment according to the invention, a speed controlled oil pump is located in the supply line for admission of pressure to the balancing piston. Depending on the result of comparison of the three-position controller, the speed of the oil pump will be increased, when the pressure at the pressure-measuring point lies below the calculated value of the regulating device, or will be decreased further, when the pressure at the pressure-measuring point exceeds the calculated value of the regulating device.

The accompanying drawing shows in:

FIG. 1 an arrangement according to the invention with a pressure-regulating valve controllable by an input signal

FIG. 2 another arrangement according to the invention with a pressure-regulating valve controllable by a three-position controller.

The screw compressor according to the arrangement pursuant to the patent (FIG. 1) is driven at the drive-shaft end 5 forming a fixed part of the male rotor 2 via a coupling not shown. The interlobe spaces of the male rotor 2 and of the female rotor 3 form working chambers to which on the suction side adjoin inlet ports 6. Due to rotation of the rotors at the drive-shaft end 5, the volume of an interlobe space considered changes. Radial bearings 1 are arranged on the suction side of the shaft shoulders, while radial bearings 9 and axial bearings 10 are arranged on the discharge side of the shaft shoulders. For compensation of the axial thrust on the male rotor 2 exerted by the gas force due to compression of the working fluid, a rotating disk, the balancing piston 11, is arranged sealing hydraulically and contactlessly at its external diameter. On one side, it is loaded with pressurized oil directly coming from the oil separator arranged on the discharge side, or the oil is brought up to a higher pressure by an oil pump. On the other side of the balancing piston, there is nearly suction pressure. Thus, the force of the balancing piston 11 counteracts the gas force on the male rotor 2 in axial direction. As a result, the axial bearings 10 are unloaded. According to the embodiment of the invention shown the regulating device 12 is arranged as part of a controlled system. The control algorithm calculates the pressure on the balancing piston 11 depending on the pressure on the compressor suction side and on the torque at the drive-shaft end 5 (essentially from motor current in connection with both motor voltage and speed entered). Pressure and motor current are passed over to the control at the interfaces 13, 14. The regulating device 12 delivers the output signal 16 for the proportional pressure-regulating valve 17. According to the control characteristics of the proportional pressure-regulating valve 17, there is a proportional relationship between the input signal and the pressure at the outlet of the proportional pressure-regulating valve 17. It increases the pressure on the balancing piston 11 in case the regulating device 12 has calculated this from the algorithm and given a corresponding output signal. It decreases the pressure on the balancing piston 11 when the regulating device 12 calculates a lower pressure for the balancing piston.

In the arrangement according to the invention (FIG. 2), a pressure-measuring point 18 is located in the supply line between the proportional pressure-regulating valve 17 and the connection for admission of pressure to the balancing piston. The output signal of the regulating device 12 and the output signal 19 of the pressure-measuring point 18 are led to a three-position controller preferably arranged in the regulating device 12 and compared. Depending on the result of the comparison, the proportional pressure-regulating valve 17 will be opened more, when the pressure at the pressure-measuring point 18 lies below the calculated value of the regulating device 12, or will be closed more, when the pressure at the pressure-measuring point 18 exceeds the calculated value of the regulating device 12. 

1. Arrangement on an oil-flooded screw compressor with two rotors, a male rotor having essentially convex lobe flanks and a female rotor having essentially concave flank portions, with the male rotor having a drive-shaft end and an oil-pressure loaded balancing piston supplied with oil via a pressure line, both rotors are enclosed in housing sections, with a regulating device arranged either on or in the compressor the inputs of which are at least connected to measuring means for the pressure on the suction side and measuring means for determination of the input power, wherein means for changing the pressure in the pressure line to the balancing piston are arranged with said means having a control interconnection to the regulating device, with the latter having an algorithm between operating parameters representing input variables for the regulating device and the pressure in the pressure line to the balancing piston.
 2. Arrangement on an oil-flooded screw compressor according to claim 1 with a regulating device wherein a pressure-regulating valve is arranged in the oil-supply line to the balancing piston, with control interconnection between the regulating device and the pressure-regulating valve, with the regulating device having an algorithm containing a relationship between the drive torque at the drive shaft of the compressor, the pressure on the compressor suction side and the pressure on the balancing piston.
 3. Screw compressor according to claim 1 wherein the relationship of the control algorithm is designed so that the pressure on the oil-pressure loaded balancing piston will increase when the driving torque rises or when the pressure on the compressor suction side rises or when in combination of rising driving torque and rising pressure on the suction side there will occur changes.
 4. Screw compressor according to claim 1 wherein the relationship of the control algorithm is designed so that the pressure on the oil-pressure loaded balancing piston will decrease when the driving torque decreases or when the pressure on the compressor suction side decreases or when in combination of decreasing driving torque and decreasing pressure on the suction side there will occur changes.
 5. Oil-flooded screw compressor according to claim 1, wherein at least one output of the regulating device has a control interconnection to the pressure-regulating valve.
 6. Screw compressor according to claim 5, wherein the output signal of the regulating device modulates the pressure on the oil-pressure loaded balancing piston.
 7. Screw compressor according to claim 1 wherein a pressure-measuring point is located in the supply line between the proportional pressure-regulating valve and the connection for admission of pressure to the balancing piston the output signal of which is connected to a three-position controller.
 8. Screw compressor according to claim 7, wherein the output signal of the regulating device is set value and the output signal of the pressure-measuring point is actual value, with both values being inputs to the three-position controller, and the output of the three-position controller has a control interconnection to the pressure-regulating valve.
 9. Screw compressor according to claim 1 wherein a speed-controlled oil pump is arranged in the supply line for admission of pressure to the balancing piston with the speed of the pump depending on the result of comparison of the three-position controller. 